The invention relates generally to electronic control circuits for electric consumers and, in particular, to a current regulating circuit for an electromagnetic consumer of a motor vehicle control unit.
Internal combustion engines with externally supplied ignition are increasingly being equipped with intermittently functioning injection systems. The fuel metering is effected by means of controlled opening times of electromagnetically actuated injection valves. This can be attained by means of high switching currents, among other provisions, at least at the instant the magnetic valve switches on. For reasons having to do with power loss, it has long been known to send a high current through the magnetic coil at the beginning of the opening period of an electromagnetic injection valve, and then to reduce this current down to the so-called maintenance current, until at the end of the injection pulse the maintenance current is switched off as well. Now in order to be able to set the lowest possible maintenance current, regulation of this current is necessary. This purpose is served by a current measuring resistor in series with the exciter winding of the magnetic magnetic valve and by a current control device (as a rule, in the form of a transistor), and the current through the magnetic winding can then be set to a specialized value via the voltage drop at the measuring resistor. Thus in the known electrical circuit layout, three components are located between the two battery voltage terminals: the magnetic winding of the injection valve, the current control device, and the measuring resistor. Particularly with a view to simple systems, the measuring resistor is not desirable, because as a rule it must be designed for high outputs at a low resistance value and thus is of no small importance in calculating the cost of the circuit layout.